Because there had to be discussions about turning the tree of life into the Avatar tree. There had to have been. I don't see James signing off on something where there were zero discussions about the size and scope of the project and what that would include.
The whole reason JK didn't sign the deal with Disney was because they told her what they wanted to do, and what they wouldn't do. They might not have had artwork or models made up, but they would have had extensive discussions about what the land would be and what it wouldn't. Why would James agree to something that wasn't talked about and agreed to?
I just don't see all of the problems with the project lying with James Cameron, like its being made out to be. I don't see him agreeing to something and then trying to take over the park after the deal is signed. And again, I don't mean to belabor the point, but everything that's been said since this thing was announced was that Avatarland was limited to a "land" with three attractions.